A speaker refers to a device that converts an electric signal into an acoustic signal. The speaker operates in such a manner that, when an electric signal is applied to a solenoid coil included in a casing of the speaker, electromagnetic field is formed in the solenoid coil and a permanent magnet around which the solenoid coil is wound. At this time, a diaphragm one side of which faces the permanent magnet vibrates to oscillate the surrounding air to convert the electric signal into an acoustic signal, thereby generating a sound.
FIG. 6 shows various types of speaker units. The speaker unit is divided into an acoustic suspension speaker unit 60a and a reflex speaker unit 60b in terms of external shape. The acoustic suspension speaker unit is constructed in such a manner that the entire face of its enclosure 62a, excepting a hole (not shown) formed in its speaker 61a, is closed for the external environment and a sound-absorbing material is attached onto the inner side of the enclosure 62a to prevent sounds generated from the inside of the speaker unit from being emitted to the outside.
The reflex type speaker unit, generally called a bass reflex or out-of-phase speaker unit, has a duct 63 formed at the front or back side of its enclosure 62b, distinguished from the acoustic suspension speaker unit. The reflex speaker unit discharges compressed air, generated caused by motion of a diaphragm (not shown), through the duct 63 to the outside and generates a sound according to reaction to the outflow of the compressed air. Here, an acoustic phase difference generates when the sound generated behind the speaker is emitted through the duct 63 so that the phase of the sound generated behind the speaker coincides with the phase of the sound emitted from the front of the speaker.
In both of the acoustic suspension and reflex speaker units, their diaphragms generate sounds having the same intensity but opposite phases in front of and behind the speaker. Frequency characteristic of sound is varied when the sound generated behind speaker interferes the sound generated in front of the speaker. The enclosure effectively prevents the interference or copes with it.
FIG. 7 is a cross-sectional view of a conventional speaker unit. The conventional speaker unit such as an earphone emits sound waves generated behind its diaphragm and compressed air generated when the sound waves are created through a plurality of fine holes 1a and 1b formed at the back side of a casing of the speaker unit to the outside. That is, the conventional speaker unit employs a reflex speaker that discharges sound waves to the back of the casing, to emit the sound waves and compressed air through the holes 1a and 1b. At this time, the compressed air is transmitted to the eardrum of a user to fatigue the user's ears. Furthermore, due to the compressed air and sound waves generated from the back of the diaphragm, a sound different from the original sound is transmitted to the eardrum in terms of characteristic of the conventional two-channel earphone. Moreover, only a short sound field exists between the eardrum and the earphone to transfer unnatural sounds to the listener.